1. Field of the Invention
The present invention relates to a method and apparatus for converting ultrasonic echo signals, occurring in succession in the form of image vectors, into image signals which are utilized for representing the scanned subject on a monitor in a format which is geometrically correct and which conforms to standards necessary for television representation, the image vectors being obtained in ultrasonic scans, and the ultrasonic scans respectively exhibiting a defined geometric form with the image vectors being intermediately stored in an image memory.
2. Description of the Prior Art
Echo signals obtained by ultrasonic scanning techniques may be in the form of parallel or non-parallel image vectors such as, for example, in sector format. Such signals are obtained by means of an ultrasonic head which may operate according to the parallel scan method (electronic or mechanical sector scan) or according to the trapezoidal scan method.
If the ultrasonic sector scan method is utilized, it is preferable to reproduce the image information generated thereby in a geometrically correct fashion in the form of image vectors displayed in a fan-shaped pattern, that is, a non-parallel pattern, in a homogenous image on the viewing screen of a television monitor. As used herein the term "image vector" is the echo information received along an ultrasonic line after emission of an ultrasonic pulse. After corresponding processing, the information is in digital form as a "one-dimensional vector." In the sector scan method, various angular directions proceeding from a common point for each of the vectors exist for various scanning times. For a so-called "real time" display conforming to television standards, it is thus necessary to employ a scan converter with an intermediate memory. The intermediate memory generally consists of a two-dimensional image memory as well as a one-dimensional "vector memory" for the purpose of writing-in or inscription, and a one-dimensional "horizontal memory" for the purpose of reading out, both of which are normally operated according to the alternating buffer principle.
In conventional methods and devices, such storage is generally undertaken with digital semiconductor memories. A scan converter is disclosed in U.S. Pat. No. 4,245,250 for an ultrasonic sector scanner which has the objective of attaining a frame repetition rate which is as high as possible. The scan lines generated by the sector scanner ("phase array") are therein scanned with varying frequency. The ultrasonic echo signals or scanning data thus acquired along a scan line are written into one of four main memories. Pre-connected time delay devices serve as intermediate memories. Each of the main memories, realized in two-dimensional form, is equipped for accepting one-forth of the complete ultrasonic echo image. Through application of the variable frequency during scanning, a "data reduction" is effected during inscription into the main memories.